Asian Erosion

In trying to interpret the marine sedimentary record it is often important to also consider the evolution of drainage systems on shore if the full sediment transport system is to be understood. In Ladakh, northern India for example, the Indus Molasse Group has been identified as the deposits of a Paleo-Indus River that was eroding western Tibet shortly after India-Asia collision in the Early Eocene. It is this system that is considered to be providing grains from the suture zone to the Arabian Sea by the Middle Eocene (~45 Ma; Clift et al., 2001). Although the age control in continental sediments is often poor compared to their marine equivalents, they form an important part of the system. Similarly, study of the modern river sediments and comparison with marine sediment allows the volume and composition of inputs from different sources to be quantified. If the process of marine sedimentation can be related to the modern mountains and drainage system then there is hope that ancient sediments can be used to reconstruct what the mountains looked like in the geologic past.

If sediment budgets derived from offshore accumulation rates are to be used to reconstruct changing continental erosion rates through time, and eventually date Tibetan Plateau uplift, then the source of that sediment needs to be constrained. Basic petrography and mineralogy are fundamental to this process, yet modern single grain probe techniques can also play an important role. I have been exploring the possibilities of the Pb isotope approach, which is based on the distinct isotopic character of the principle source regions. Analysis is made using detrital K-feldspars, a techniques pioneered by conventional mass spectrometry (e.g., McDaniel et al., 1994). It has already been shown that the modern rivers of SE Asia and the tributaries within the Indus system carry K-feldspar grains with resolvably different Pb isotope characters (Bodet & Schärer, 2001; Clift et al., 2002). I have been working with colleagues at Woods Hole Oceanographic Instution (USA) and in Stockholm (Sweden) employ the newly developed technique of measuring Pb from single grains in situ (Layne & Shimizu, 1998) using a high resolution Cameca 1270 ion microprobe. Used in company with bulk sediment techniques, such as Nd isotopes, we are able to mass balance erosion in the mountains with deposition along the continetal margin.

Uplift of the Tibetan Plateau has caused an increased flux of clastic material from Asia into the surrounding marginal seas. This flux reflects increased regional erosion, partially caused by topographic uplift of Tibet and the incision of gorges into the eastern flank of the plateau, but dominantly by higher precipitation linked to monsoon intensification. Using regional seismic profiles from East Asia as proxies for the Cenozoic accumulation history Clift et al. (2004a) have constrained the timing of enhanced erosion and thus climate change and plateau uplift. Most of the seas show sedimentation increasing after ~45 Ma following India-Asia collision. Rates rose sharply at ~33 Ma, and again around 18 Ma, possibly caused by stages in monsoon strengthening enhancing regional erosion coupled with phases of plateau uplift. Erosion of gorges in eastern Tibet appears to be an insignificant source of sediment to the marginal seas. The late Miocene (5–11 Ma) is a time of reduced sedimentation in all basins except in the Mekong area, which alone is influenced by exhumation of the Vietnamese Central Highlands. The slower regional erosion reflects a drier climate and strong winter monsoon during the late Miocene, while the Plio-Pleistocene (<5 Ma) sees a return to fast accumulation rates, possibly triggered by global climatic deterioration.

The marine sedimentary record can be used to reconstruct the erosion of the Himalayas and Tibet since the start of India-Asia collision. This record is essential to any attempts to quantify the relationships between tectonics, climate change and erosion. In particular, understanding the relationship between Tibetan uplift and monsoon strengthening and its erosional response is still unresolved because of uncertainty about the timing of each process. Prior work by Clift now suggests that the plateau uplift may be much older than previously believed 16-20 Ma, rather than 8 Ma, and that this caused a strengthening of the monsoon across east and south Asia, increasing erosion and marine clastic sedimentation in the marginal seas. Future work is now aimed at dating key drainage capture events and in constructing robust sediment budgets in the marginal seas using regional seismic profiles and drilling data from industrial and future IODP sources.

Clift, P.D., Campbell, I.H., Zhang, X., Carter, A., Hodges, K.V., Khan, A.A., and Allen, C.M., 2004b, Thermochronology of the modern Indus River bedload: New insight into the controls on the marine stratigraphic record. Tectonics, in press.

Clift, P.D., Layne, G.D., and Blusztajn, J. 2004a, The erosional record of Tibetan uplift in the East Asian marginal seas, In: Continent-Ocean Interactions in the East Asian Marginal Seas, edited by Clift, P.D., Wang, P., Hayes, D., and Kuhnt, W., American Geophysical Union, monograph in press.

Clift, P. D., 2002. A brief history of the Indus River. In, Clift, P.D., Kroon, D., Craig, J., and Gaedicke, C. (Editors), The Tectonic and Climatic Evolution of the Arabian Sea Region, Geological Society of London special publication, 195, 237–258.

Clift, P.D., Lee, J. I., Blusztajn, J. and Clark, M.K., 2002. Erosional response of South China to arc rifting and monsoonal strengthening recorded in the South China Sea. Marine Geology, 184, 207–226.

Clift, P. D., Lee, J.I., Hildebrand, P., Shimizu, N., Layne, G. D., Blusztajn, J., Blum, J. D., Garzanti, E., and Khan, A. A., 2002. Nd and Pb isotope variability in the Indus River system: Implications for sediment provenance and crustal heterogeneity in the Western Himalaya. Earth and Planetary Science Letters, 200, 91–106.

Clift, P.D. and Gaedicke, C., 2002. Accelerated mass flux to the Arabian Sea during the Middle-Late Miocene. Geology, 30, 207–210.

Clift, P.D., Shimizu, N., Layne, G., Gaedicke, C., Schlüter, H.U., Clark, M. and Amjad, S., 2001. Development of the Indus Fan and its significance for the erosional history of the western Himalaya and Karakoram. Geological Society of America Bulletin, 113, 1039–1051.

Clift, P.D., Shimizu, N., Layne, G., and Blusztajn, J., 2001. Tracing patterns of unroofing in the Early Himalaya through microprobe Pb isotope analysis of detrital K-feldspars in the Indus Molasse, India, Earth and Planetary Science Letters, 188, 475–491.